The present Applicant has developed a plethora of high-speed inkjet printers employing stationary Memjet® printheads which extend across a media width. By contrast, virtually all other types of inkjet printer utilize a scanning printhead which traverses across the media width.
High-speed pagewidth printing necessarily places additional demands on the design of the printhead compared to traditional types of inkjet printhead. The nozzle devices must have a self-cooling design, high ink refill rates and high thermal efficiency. To this end, the Applicant has developed a range of thermal bubble-forming printheads, including those with suspended resistive heater elements (as described in, for example, U.S. Pat. Nos. 6,755,509; 7,246,886; 7,401,910; and 7,658,977, the contents of which are incorporated herein by reference) and those with embedded (“bonded”) resistive heater elements (as described in, for example, U.S. Pat. Nos. 7,377,623; 7,431,431; US 2006/250453; and U.S. Pat. No. 7,491,911, the contents of which are incorporated herein by reference).
Nozzle devices having suspended heater elements offer the advantages of efficient heat transfer from the heater element to the ink and self-cooling characteristics. However, they suffer from the disadvantage of relatively short printhead lifetimes, because suspended heater elements are typically less robust than their bonded counterparts.
One approach to improving printhead lifetime is to coat the heater elements with a layer of protective coating. For example, U.S. Pat. No. 6,719,406 (assigned to the present Applicant) describes suspended heater elements having a conformal protective coating, which improves the robustness of the heater element and improves printhead lifetime. However, protective coatings are undesirable for a number of reasons—they reduce the efficiency of heat transfer from the resistive heater elements to the surrounding ink; they consequently affect the self-cooling characteristics; and they introduce additional MEMS fabrication challenges.
Therefore, it is generally preferable to employ uncoated (“naked”) heater elements in Memjet® printheads with a consequential reduction in printhead lifetime. To some extent, the choice of heater material can mitigate the effects of using uncoated heater elements. For example, U.S. Pat. No. 7,431,431 describes the use of a self-passivating titanium aluminium nitride heater element, which has improved lifetime compared to more conventional materials, such as titanium nitride. Nevertheless, there is still a need to improve the lifetimes of Memjet® printheads, and particularly those employing uncoated heater elements.
It has been found that certain inks are particularly aggressive towards heater elements. For example, many dye-based inks have been found to corrode heater elements resulting in shortened printhead lifetimes. In a multi-color printhead (e.g. CMYK), the printhead lifetime is, to a large extent, limited by the lifetime of the color channel having the shortest lifetime. If, for example, a black dye-based ink is found to be particularly corrosive towards heater elements, then the lifetime of the printhead will be determined by the lifetime of the black channel, even if all other color channels still perform well when the black color channel fails.
In the present context, “failure” of a nozzle device means any change in drop ejection characteristics which results in unacceptable print quality. For example, failure may be invoked by a reduction in drop velocity, poor drop directionality or non-ejection of ink. Moreover, the criteria for failure may be different for different colors. For example, a reduction in print quality in a yellow channel may be more tolerable than a corresponding reduction in print quality in a black channel, because black ink is more visible to the human eye (i.e. black ink has a higher luminance on white paper). This, in combination with the aggressive nature of many black dyes, means that the black channel in a Memjet® printhead is typically the limiting color channel in terms of printhead lifetime.
US2012/0090503 describes a number of disazo dyes formulated in ink vehicles comprising ethylene glycol.
WO2015/054682 describes certain ink formulations comprising a black disazo dye, sulfolane and triethylene glycol having improved printhead lifetimes.
However, ethylene glycol and sulfolane are harmful solvents, which potentially limits their use in some applications. Under the Globally Harmonized System of Classification and Labelling of Chemicals (GHS), sulfolane is classified as GHS07 (“Exclamation Mark”) while ethylene glycol is classified as GHS08 (“Health Hazard”).
It would be desirable to improve printhead lifetimes without modifying the design of printheads and without the use of potentially harmful solvents.